Various sensors utilizing optical fibres are known. Many such sensors rely on fibre optic point sensors or discrete reflection sites such as fibre Bragg gratings or the like being arranged along the length of an optical fibre. The returns from the discrete point sensors or reflection sites can be analysed to provide an indication of the temperature, strain and/or vibration in the vicinity of the discrete sensors or reflection sites.
Such sensors using discrete reflection sites or fibre optic point sensors require the optical fibre including the sensor portions to be specially fabricated. Further the distribution of the sensors within the optical fibre is fixed.
Fully distributed fibre optic sensors are also known in which the intrinsic scattering from a continuous length of optical fibre is used. Such sensors allow use of standard fibre optic cable without deliberately introduced reflection sites such fibre Bragg gratings or the like. The entire optical fibre from which a backscatter signal can be detected can be used as part of the sensor. Time division techniques are typically used to divide the signal returns into a number of time bins, with the returns in each time bin corresponding to a different portion of the optical fibre. Such fibre optic sensors are referred to as distributed fibre optic sensors as the sensor options are fully distributed throughout the entire optical fibre. As used in this specification the term distributed fibre optic sensor will be taken to mean a sensor in which the optical fibre itself constitutes the sensor and which does not rely on the presence of specific point sensors or deliberately introduced reflection or interference sites, that is an intrinsic fibre optic sensor.
Various types of distributed fibre optic sensor are known and have been proposed for use in various applications.
U.S. Pat. No. 5,194,847 describes a distributed acoustic fibre optic sensor for intrusion sensing. A continuous optical fibre without any point sensors or specific reflection sites is used. Coherent light is launched into the optical fibre and any light which is Rayleigh backscattered within the optical fibre is detected and analysed. A change in the backscattered light in a time bin is indicative of an acoustic or pressure wave incident on the relevant portion of optical fibre. In this way acoustic disturbances any portion of the fibre can be detected.
GB patent application publication No. 2,442,745 describes a distributed acoustic fibre optic sensor system wherein acoustic vibrations are sensed by launching a plurality of groups of pulse modulated electromagnetic waves into a standard optical fibre. The frequency of one pulse within a group differs from the frequency of another pulse in the group. The Rayleigh backscattering of light from intrinsic reflection sites within the fibre is sampled and demodulated at the frequency difference between the pulses in a group.
Distributed fibre optic sensing or distributed acoustic sensing (DAS) therefore provides useful and convenient sensing solutions that can monitor long lengths of optical fibre with good spatial resolution. For instance a distributed fibre optic acoustic sensor, for instance as may be used for monitoring a pipeline, can be implement with sensing portions 10 m long in up 40 km or more of optical fibre.
Each sensing portion can detect any incident acoustic disturbances and such sensor have been proposed for use in intrusion detection systems, condition monitoring systems, seismic surveying and operational monitoring, i.e. monitoring the operation of some apparatus. However a distributed acoustic sensor such as described in GB 2,442,745 basically indicates whether there is an acoustic disturbance at a particular section of the sensing fibre. This provides an indication of where along the fibre an acoustic event has occurred but it gives no information as to the point of origin of the acoustic disturbance in relation to the fibre, i.e. there is no information regarding the lateral offset of the origin of the acoustic disturbance from the fibre—how far away from the fibre and/or in which direction.